A single-phase power conditioning device as a conventional power inverter device for converting a DC power of a solar cell to an AC power is disclosed in, e.g. PTL 1.
FIG. 7 is a schematic view of power system 100 including power conditioning device 104 disclosed in PTL 1. Power conditioning device 104 is a single-phase power conditioning device. Power system 100 includes energy source 102 and power conditioning device 104 connected to AC load 106 and energy storage section 108 AC load 106 may be single phase-connected to an AC power grid. Energy source 102 is, e.g. a solar cell. Energy storage section 108 is an electricity storage element, such as a capacitor. A control device integrated with power conditioning device 104 functions to maintain the operation of power system 100 at an optimal power point so that a maximum power can be output from energy source 102 whenever power is available. Power conditioning device 104 is, e.g. an inverter.
An operation of power system 100 including power conditioning device 104 will be described below. Power system 100 detects a first AC waveform based on the output from power conditioning device 104. Then, a second AC waveform having the same frequency as that of the first AC waveform is generated by energy storage section 108. The second AC waveform has a phase shifted by π/4 radian from the phase of the first AC waveform. This operation can consequently minimize a double-frequency ripple power of the power supplied from energy source 102. Specifically, electric energy having the second AC waveform flowing into or out of energy storage section 108 is controlled by the switching operation of an interface section provided between power conditioning device 104 and energy storage section 108. The shifted second AC waveform is tracked to compensate the ripple power having a double frequency.
Power system 100 including power conditioning device 104 (inverter) can reduce the double-frequency ripple power of the power supplied from energy source 102. However, the stable reduction of the ripple power may require a period of time of several tens of seconds during which the power is unstable, hence adversely influencing the operation of AC load 106.